You should apply to your local council as soon as you need help with your rent, because Housing Benefit usually starts from the date you claim. Backdating may be possible if you had a good reason for not applying sooner: the usual maximum is one month for working-age claimants and up to three months for people who have reached State Pension age, subject to the council’s rules. Most working-age tenants now claim Universal Credit instead, although exceptions include some supported or temporary accommodation. Check with your council or the relevant GOV.UK guidance to confirm which benefit and deadlines apply to you.